One form of gene expression impairment by RNA-RNA duplex formation has been termed "antisense" inhibition. Exploitation of antisense gene regulation could lead to potent anti-viral therapy. A serious limitation of the antisense approach, especially as it applies to anti-viral activity, is that it is stoichiometric and may require large molar excesses of anti-sense versus target RNA to be effective.
Within the last few years, discoveries of ribozymes, e.g., RNAs with enzymatic activities have led to the development of antisense molecules which not only form RNA-RNA hybrids, but catalytically cleave the covalent phosphodiester linkages and turnover large numbers of substrate molecules. Ribozymes can now be targeted to virtually any RNA transcript, and efficient cleavage can be readily achieved in vitro. See, Kim, S. H., et al. Proc. Natl Acad. Sci. U.S.A. 84:8788-8792 (1987); Haseloff, J., et al., Nature 234:585-591 (1988); Cech, T. R. JAMA 260:3030-3034 (1988); Jeffries, A. G., et al., Nucleic Acids Research 17:1371-1377 (1989).
Haseloff and Gerlack developed a set of rules useful to design transacting ribozymes. The utility of these rules was demonstrated by ribozymes effectively targeted to several different sites within the chloramphenical acetyltransferase transcript. Haseloff states that "A major potential application for these highly sequence specific endoribonucleases is in cleavage and thereby inactivation of gene transcripts in vivo . . . . Provided that the transcribed sequences of the gene are known, it should be possible to target one or more ribozymes against specific RNA transcripts. Expression in vivo of such ribozymes and cleavage of the transcripts would in effect inhibit expression of the corresponding gene. This `auto-gene` activity of the ribozymes could provide a basis for various gene and viral therapies and analyses" (pp. 590-591).